The present invention relates to a method of sensor packaging providing a human contact interface and, more particularly, to a flexible sensor packaging that protects embedded integrated circuitry and/or electronic devices from damage resulting from human contact force or pressure.
The measurement of a human contact force applied to a small area, which is to say, pressure, especially the pressure applied by the touch of a thumb or finger to a sensor that is sensitive to mechanical excitation, is of particular interest in applications in which small-size devices, such as cellular telephones, games, toys, and other consumer electronics controls, are desirable for converting the applied force into an electrical value.
For effective and reliable operation, a tactile pressure-sensing chip must satisfy two design requirements. First, the sensing chip must provide an efficient conversion of the applied pressure into some degree of mechanical excitation. Second, the sensing chip must protect the integrated circuitry and other embedded components from damage that may result from the induced mechanical stress. For example, membrane-type pressure-sensing chips that are embedded in conventional packaging have been fabricated and used for a long time. However, to avoid damage to the bonding pads and/or to the wire bonding that surround the chip's membrane, the protective material must be relatively hard. As a result, current pressure-sensing chips often require significant force from a human touch.
FIG. 1 shows conventional sensor packaging such as that disclosed in U.S. Pat. No. 6,401,545 to Monk, et al. According to the teachings of Monk, et al., a pressure sensor 1 is attached to a support 2 using an adhesive 3. The sensor 1 includes a pressure-sensitive membrane 4 and electrical contact pads 5, which are embedded in the pressure sensor 1 substrate. Bonded wires 6 electrically couple the sensor's electrical contact pads 5 to the support's bonding pads 7.
On an upper surface of the sensor 1, a dam 9 is fabricated around the periphery of the pressure-sensitive membrane 4. A first material 10, e.g., a gel, is deposited on top of the membrane 4, within the area confined by the dam 9. The mechanical properties of the first material 10 transfer pressure applied to the surface of the first material 10 to the membrane 4. The second material 11, e.g., a gel, is deposited outside of the limits of the dam 9, to cover and encapsulate the contact pads 5, the bonding pads 7, and the bonding wires 6, to protect them from the external environment, e.g., moisture, dust, temperature, light, and so forth.
Disadvantageously, the pressure-sensitive sensor 1 taught by Monk, et al. requires additional, specific fabrication steps at the sensor level, for example, to fabricate the dam 9, to embed the pressure-sensitive membrane 4 and electrical contact pads 5 in the substrate of the sensor 1, and to deposit the first and second materials 10 and 11, respectively, within and outside the limits of the dam 9.
FIG. 2 shows a force-type sensor using a capacitive-based pressure-sensing chip 13 of a type disclosed in U.S. Pat. No. 7,148,882 to Kamrath, et al. According to the teachings of Kamrath, et al. the capacitive-based pressure-sensing chip 13 is disposed on an upper surface of a substrate 12. Three discrete spacing layers 16, 17, and 18 surround the chip 13. A deformable membrane 14 is disposed on top of the upper spacing layer 17 and the plenum 19 created between the deformable membrane 14 and the upper surface of the substrate 12 is filled with a fluid.
Disadvantageously, the capacitive-based pressure-sensing chip 13 of Kamrath, et al. requires placement of three spacing layers 16, 17 and 18, complicating mass production fabrication. The Kamrath, et al. sensor 12 also does not provide protection to the capacitive-based pressure-sensing chip 13 except through the bending (flexural) resistance of the membrane 14.
Other embodiments of human contact-pressure sensor devices in the prior art include pressure-sensitive organic materials, which feature electrical sensitivity to an applied force. However, such organic materials are more expensive than silicon materials, which are a material of choice in MEMS and the integrated circuit industry.
However, in short, the prior art has failed to combine the advantages of a low fabrication cost, efficient conversion between external force load transfer to the sensor, and protection of the electrical circuitry and other embedded devices. Therefore, it would be desirable to provide a pressure-sensitive sensor that integrates a relatively soft material above the pressure-sensitive area, to effect a direct mechanical transfer from an externally-applied load to the pressure-sensitive area, with a relatively hard material, to protect the bonding wires, contact pads, bonding pads, and the like. Moreover, it would be desirable to provide silicon-based pressure sensors as well as any other standard or non-standard devices having small-force sensitive areas.